Comparison picture of video screen sizes http://forums.adobe.com/thread/1120039
Going from HD down to SD results in the loss of a LOT of screen size
Edit in Premiere Pro at the actual size of your HD video, export as MPEG2 DVD
I export AVCHD to Widescreen DV AVI to then let Encore do automatic transcoding for a "best fit" of video length to DVD size (single or dual layer) with excellent results http://forums.adobe.com/thread/652694
If you provide more information about versions and video, someone else may have another idea - Information FAQ http://forums.adobe.com/message/4200840